The present invention relates to a ventilatory support system, and more particularly relates to a method and apparatus for providing a controlled flow of breathing gas to a patient based on the function of the patient""s upper airways which vary over time in relation to the physiological requirements of the patient.
There are many clinical disorders that are characterized by the failure of patients to maintain adequate ventilation. In general, ventilation can be compromised in one of two ways. First, there are patients who cannot breathe adequately due to excessive mechanical loads on the ventilatory apparatus or weakness of the respiratory muscles. Examples of such mechanical loads include upper airway obstruction in obstructive sleep apnea, bronchial obstruction in asthma and chronic obstructive pulmonary disease, and reductions in lung or chest wall compliance in diseases involving the pulmonary parenchymal and chest wall. Second, ventilation may be compromised by a failure of neuromuscular mechanisms in patients who may have disorders involving the central nervous system or phrenic nerves. Regardless of etiology, each of these disorders is associated with reduced levels of ventilation.
In patients with reduced levels of ventilation, ventilation can be augmented by blowing air into the airway. In one approach, air can be applied to help wash CO2 out from the airways. When an additional source of air is applied to the central airways, ventilation may fall because less is required to eliminate CO2. Currently, two such methods are utilized clinically to aid CO2 washout from the airways. In intubated patients, air is administered either continuously or during expiration by a process known as tracheal gas insufflation (TGI). Alternatively, air can be administered through a thin transtracheal cannula in non-intubated, spontaneously breathing patients. Current evidence suggests that low flow rates up to 5 to 6 liters/min can wash out CO2 and reduce a patient""s ventilatory requirements. For CO2 washout to occur, insufflated air must vent freely to atmosphere. With continuous transtracheal insufflation (TTI), therefore, CO2 washout allows patients to reduce ventilation without increasing CO2.
In another approach, mechanical ventilation can be instituted to augment ventilation. Positive pressure ventilation is the most common form of mechanical ventilation. It is characterized by intermittent application of positive pressure to the airway. When airway pressure is increased, the lungs inflate. Deflation occurs passively after allowing the airway pressure to fall. Therefore, positive pressure can be applied intermittently to the airway to augment ventilation in patients who cannot maintain normal levels of ventilation on their own. Various mechanisms have been developed to augment ventilation with positive pressure devices, including endotracheal tubes, tracheostomy tubes and nasal/oronasal masks. In each example, a tight seal is required between the ventilator and the patient""s airway, thereby preventing leakage of air when positive pressure is applied. As a result, these interfaces are relatively intrusive, and interfere with speech, swallowing, normal breathing patterns, and normal sleep/wake rhythms.
Obstructive sleep apnea is one example of a condition in which there is cyclic occlusion and reopening of the pharynx, which results in the obstruction of airflow during sleep, hypoxic episodes and daytime somnolence. Two general approaches have been utilized to treat this disorder. First, methods have been devised to relieve pharyngeal airflow obstruction. At present, nasal continuous positive airway pressure (nCPAP) is the most effective way to relieve obstruction. It is applied via a nasal mask and maintains pharyngeal patency during sleep. CPAP is most effective when a tight seal is maintained between the patient""s airway and the nasal mask. U.S. Pat. Nos. 5,551,419, 5,540,219, Re. 35,295, 5,535,738, and 5,490,502 disclose the use of such CPAP devices. However, despite its low incidence of side effects CPAP is often not well tolerated, and many patients do not adhere to therapy because the tightly applied nasal mask causes claustrophobia (Kribbs et al., Am. Rev. Respir. Dis., Vol. 147, 1993). The present invention, however, does not require such a tight seal. Rather than relieving upper airway obstruction as nasal CPAP, it works in concert with the patient""s breathing efforts and the natural tendency of the upper airway to collapse and obstruct the exit of airflow from the lungs.
When relief of pharyngeal obstruction is not achievable with nasal CPAP, tracheostomy provides an alternative breathing route during sleep which bypasses the pharynx. It is effective in treating this disorder because it provides a widely patent bypass route for breathing. With tracheostomy, air can be inspired and expired freely from/to atmosphere irrespective of the state of the upper airway patency. Although highly effective in treating apneic patients, tracheostomy is associated with significant morbidity from repeated airway infections, intractable cough, speech difficulties and disfigurement. Because of its high morbidity, tracheostomy is rarely considered by either patients or physicians to be an acceptable therapeutic alternative, except when sleep apnea is life-threatening. The present invention avoids these adverse effects, yet provides a mechanism for both inspiration and expiration that utilizes the upper airway to coordinate the pattern of airflow.
Another proposed method is to provide long-term supplemental oxygen therapy via a thin transtracheal cannula through which a low flow rate of oxygen is delivered intratracheally to patients with lung disease. U.S. Pat. Nos. 5,181,509 and 5,090,408 disclose examples of such cannulas. Clinical reports and experience with this type of cannula has shown it to be an effective, well tolerated oxygen delivery method. However, the low flow rate of oxygen is not sufficient to provide satisfactory ventilatory support to patients.
U.S. Pat. Nos. 5,101,820 and 5,279,288 to Christopher disclose the use of a transtracheal catheter to provide a continuous high flow rate of oxygen-containing gas to a patient. However, there are disadvantages associated with the continuous delivery of gas to patients, such as spasm of the vocal cords and closure of the upper airway. This can result in the rapid buildup of excessive pressure in the trachea and lungs of the patient (pneumothorax and pneumomediastinum) and alterations in the breathing pattern which perpetuate problems with sleep disruption and daytime hypersomnolence.
The disclosure of each of the patents cited above is incorporated herein by reference.
The present invention has been developed in view of the foregoing and to overcome other deficiencies of the prior art.
The present invention provides a ventilatory support system which controls the flow of breathing gas to a patient based on the function of the patient""s upper airway and his or her ventilatory needs. Gas pressure in the trachea of the patient is measured, and the delivery of breathing gas to the patient is controlled based on the sensed gas pressure. Depending on the upper airway function of the patient, a tracheal gas pressure limit and a breathing gas flow rate value are established. When the tracheal gas pressure limit is reached, the flow of breathing gas is reduced or terminated. The flow of breathing gas is subsequently resumed either immediately, after a delay period, or after the tracheal gas pressure falls to a predetermined level. The system thus provides a feedback loop using tracheal pressure which reflects a patient""s ventilatory and upper airway status in order to control the flow of breathing gas.
An important feature of the present invention is that the upper airways of the patient constitute an integral part of the breathing circuit. The upper airways serve as a valve which controls whether the applied tracheal breathing gas inflates the lungs or vents to atmosphere. We have recognized that the upper airways obstruct flow progressively as the tracheal pressure falls toward and below a critical pressure (PCRIT). As the upper airway obstructs progressively, more and more of the applied flow is directed toward and inflates the lungs. Under these circumstances, a greater proportion of the breathing gas supplied by the device is used to satisfy his or her ventilatory demand. On the other hand, any increase in tracheal pressure above this critical pressure facilitates exhalation of breathing gas and CO2 washout from anatomic dead space. A major advantage of the invention is that it can maintain tracheal pressure in a range which optimizes delivery of inspired gas to the lungs and facilitates venting of exhaled gas through the upper airways.
The system of the present invention is useful in treating many different types of clinical disorders. For example, the system may be used to treat patients with upper airway obstruction, such as patients suffering from obstructive sleep apnea. The present system may be used to treat such patients by maintaining tracheal gas pressure above a critical level so as to provide sufficient air to inspire, yet allow free release of air through the upper airways during expiration. Similar mechanisms can be utilized to augment both ventilation and CO2 washout in patients with other breathing disorders, even when upper airway obstruction is absent.
In a preferred embodiment, treatment with the system of the present invention is associated with alternate opening and with partial or complete closing of the upper airway of the patient. Depending on the level of the patient""s tracheal pressure, the upper airway can be either open or closed to atmosphere. The system of the present invention controls the flow of breathing gas that it supplies to the lungs in concert with the patient""s breathing efforts, and as a function of the patient""s tracheal pressure. When tracheal pressure decreases, the upper airway progressively closes, thereby directing the flow of breathing gas toward the lungs. In contrast, when tracheal pressure increases during expiration, the upper airway progressively opens and exhaled gas is vented. The present invention interacts with the patient in such a way as to coordinate the delivery of flow to the trachea with both the patient""s breathing demand and state of upper airway patency.
An object of the present invention is to provide a method for giving interactive ventilatory support to a patient based on the patient""s ventilatory requirements and on the properties of the upper airways. A controlled flow of breathing gas is delivered to the patient based on the gas pressure in the trachea of the patient.
Another object of the present invention is to provide a method for supplying breathing gas to a patient including the steps of inserting a catheter into the trachea of a patient, establishing a tracheal gas pressure limit for the patient, measuring gas pressure in the trachea, and controlling the flow of breathing gas through the catheter based on the measured gas pressure in the trachea and the properties of the upper airways. The catheter is preferably inserted transtracheally. A breathing gas flow rate value is preferably established for the patient depending upon the upper airway properties and degree of ventilatory support required by the patient.
Another object of the present invention is to provide an apparatus for supplying breathing gas to a patient. The apparatus includes means for delivering breathing gas into the trachea of the patient, means for measuring gas pressure in the trachea, and means for controlling the flow of the breathing gas through the breathing gas delivery means into the trachea based on the measured gas pressure in the trachea.
Another object of the present invention is to provide an apparatus for supplying breathing gas to a patient including a source of breathing gas, a catheter in communication with the source of breathing gas, a tracheal pressure sensor for measuring gas pressure in the trachea of the patient, and a breathing gas flow controller connected to the source of breathing gas and the tracheal pressure sensor for controlling the flow of breathing gas.
These and other objects of the present invention will be more apparent from the following description.